It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone", the presence of the marker is detected and an alarm is generated. Some markers of this type are intended to be removed at the checkout counter upon payment for the merchandise. Other types of marker remain attached to the merchandise but are deactivated upon checkout by a deactivation device which changes a magnetic characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
One type of EAS system employs magnetomechanical markers that include a magnetostrictive element. U.S. Pat. No. 4,510,489, issued to Anderson et al., discloses a marker formed of a ribbon-shaped length of a magnetostrictive amorphous material contained in an elongated housing in proximity to a biasing magnetic element. The magnetostrictive element is fabricated such that it is resonant at a predetermined frequency when the bias element has been magnetized to a certain level. At the interrogation zone, a suitable oscillator provides an AC magnetic field at the predetermined frequency, and the marker mechanically resonates at this frequency upon exposure to the field when the bias element has been magnetized to a certain level. The interrogation field is provided in pulses or bursts. A marker present in the interrogation field is excited by each burst, and after each burst is over, the marker undergoes a damped mechanical oscillation. The resulting signal radiated by the marker is detected by detecting circuitry which is synchronized with the interrogation circuit and arranged to be active during the quiet periods after bursts. EAS systems of the above-described pulsed-field magnetomechanical type are sold by the assignee of this application under the brand name "Ultra*Max" and are in widespread use. (The disclosure of the Anderson et al. patent is incorporated herein by reference.)
In magnetomechanical markers of the type described above, the bias element may be utilized as a control element to switch the marker between activated and deactivated states. Typically, the bias element is formed of a semi-hard magnetic material, such as the material designated as "SemiVac 90", which is available from Vacuumschmelze, Hanau, Germany. Conventional bias elements are in the form of a ribbon-shaped length of the semi-hard material. To place the marker in the activated condition, the bias element is magnetized substantially to saturation with the polarity of magnetization parallel to the length extent of the bias element. To deactivate the marker, the magnetic state of the bias element is substantially changed, as, for example, by degaussing the bias element by applying thereto an AC magnetic field at a level higher than the coercivity H.sub.c of the material. When the bias element has been degaussed, it no longer provides the bias field required to cause the magnetostrictive element (also known as the "active element") to oscillate at the predetermined operating frequency of the EAS system. In addition, the level of the signal output by the magnetostrictive element is greatly reduced in the absence of the bias field. Consequently, when the bias element has been degaussed, the magnetostrictive element does not respond to the interrogation signal so as to produce a signal that can be detected by the detection circuitry of the EAS system.
Co-pending patent application Ser. No. 08/697,629, filed Aug. 28, 1996 (which has a common assignee and a common inventor with the present application), discloses an improved magnetomechanical EAS marker in which the bias element is formed of a semi-hard magnetic material which has a lower coercivity than conventional materials for bias elements. When such low-coercivity bias elements are used, it is possible to deactivate markers by applying a much lower level AC field than was required with conventional, higher-coercivity bias elements. This, in turn, allows for a reduction in the power level at which deactivation equipment is operated. Also, or alternatively, the markers can be reliably deactivated at a greater distance from the deactivation device than was feasible with higher-coercivity bias elements. Moreover, with the lower power level required for deactivation of the low-coercivity bias elements, it becomes feasible to operate deactivation equipment in a continuous wave mode, rather than in triggered pulses as has been the practice in conventional deactivation equipment.
For the reasons given above, it is desirable that magnetomechanical EAS markers be deactivatable with a rather low level AC field. However, it is a competing desirable characteristic of EAS markers that the same be "stable". That is, when a marker is in an activated condition, its response characteristics should not be adversely affected by exposure to stray magnetic fields that may be encountered during shipment, handling or storage of the marker. It will be understood that if the coercivity of the bias element is too low, the risk of unintentional deactivation by exposure to stray fields may become excessive.
The inevitable trade-off between stability and low deactivation field level can be ameliorated if the bias element exhibits "abruptness". That is, it is desirable that the bias element exhibit stability over a range of applied AC fields from zero up to a threshold level, and that the bias element exhibit a rather sharp or abrupt decrease in magnetization in response to exposure to an AC field having a peak amplitude above the threshold level.